Psoas major
The spinal half of the iliopsoas, and the only muscle joining the spine to the leg.
- Type
- Muscle
- Region
- Hip
- Standard name
- m. psoas major
- Origin
- Bodies and transverse processes of the twelfth thoracic and all lumbar vertebrae, and the discs between them
- Insertion
- Lesser trochanter of the femur, via the common iliopsoas tendon
- Actions
- hip flexionlumbar stabilisation
- Verify
- TA2 2595 TA98 A04.7.02.004
What it is
A long, thick muscle running down either side of the lumbar spine, deep inside the abdomen. It arises from the vertebral bodies and transverse processes from the twelfth thoracic vertebra downwards, and from the discs in between, then descends through the pelvis to join iliacus in the common tendon that reaches the lesser trochanter of the femur.
It sits behind the abdominal contents and in front of the hip joint, which is an awkward place to be and the reason nobody can palpate it reliably.
Some people also have a psoas minor, a thin muscle in front of it. It is absent in a substantial minority and does very little when present — one of the more common anatomical variations in the body.
What it does
Flexes the hip, powerfully, in company with iliacus. Acting from below, it bends the trunk forward on the thigh.
Its action on the lumbar spine is the interesting and contested part. Attaching along five vertebrae and their discs, it clearly acts on the lower back, but whether it increases the lumbar curve, compresses the segments, stabilises them, or does different things at different joint angles has been argued for decades and is not resolved. Descriptions that state one of these flatly are overstating what is known.
What loads it
Everything that loads iliopsoas, because in practice they cannot be separated: knee lifts against resistance, running, stairs, and holding the legs off the ground.
What is commonly got wrong about it
That it can be released, massaged or "let go". It lies behind the abdominal organs. Deep abdominal pressure aimed at it is passing through bowel, blood vessels and other structures first, and the specificity claimed for those techniques is not plausible from the anatomy.
That it is where the body stores fear. A popular claim with no anatomical or physiological basis. It is included here because it is repeated so often that its absence from a page about this muscle would be conspicuous.
That tightness here is the cause of your back pain. It might be involved. So might a dozen other things, and telling them apart needs an examination.